Accelerating Oxygen Electrocatalysis Kinetics on Metal–Organic Frameworks via Bond Length Optimization

Highlights The acid etching Co-naphthalenedicarboxylic acid-based metal–organic frameworks (donated as AE-CoNDA) catalyst displayed an excellent oxygen evolution reaction (OER) activity for long-term stability. Integration of the AE-CoNDA cocatalyst into BiVO4 achieved a remarkable PEC-OER activity. The stretched Co-O bond length regulated the spin state transition at the Co active sites. The optimized high spin state of Co sites adjusted the orbitals hybridization of Co 3d and O 2p. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-024-01382-9.

For the OER processes in alkaline, it typically involves a four-electron transfer pathway and complicated deprotonation process of oxygen-contained intermediates (e.g., OH*, O*, and OOH*) [35].Conventional MOFs materials usually contain saturated coordinated metal nodes that are unfavorable for adsorption of oxygen-contained intermediates.In addition, the framework structure of traditional MOFs is vulnerable to severe collapse under harsh condition.Thereby, it is important to modify the crystal structure of MOFs for boosting the OER activity and enhancing photoelectrode's stability.Like the natural enzyme structure [24], distorted structure of catalyst has an effect on electronic spin configuration of metal sites, which is closely associated with the binding energy for oxygen-contained intermediates on the metal sites.Compared to thermal-induced or photoinduced method, acid etching is simplicity of operation and easily control distorted degree.
In particular, a bond length adjustment strategy was represented to regulate the intrinsic electronic structure of Co-2,6-naphthalenedicarboxylic acid-based MOFs (AE-CoNDA) catalyst by acid etching treatment.The AE-CoNDA catalyst showed a superior OER performance and rapid kinetics in alkaline media, featured by a low overpotential of 260 mV to reach 10 mA cm −2 and a small Tafel slope of 62 mV dec −1 .Integration of the AE-CoNDA cocatalyst into a BiVO 4 for solar water splitting led to an AM 1.5G photocurrent density of 4.30 mA cm −2 at 1.23 V, which outperformed the PEC-OER performance for most reported Co-based BiVO 4 photoanodes (Table S1).Experimental results verified that the electronic structure on the Co active center in the AE-CoNDA was reconfigured by the bond length adjustment.An optimized bond length corresponded to the spin state transition from an intermediate spin (e g 1 t 2g 6 ) to a high spin (e g 2 t 2g 5 ) at the Co active site, facilitating the oxygen-contained intermediates adsorption.Theoretical calculations uncovered the optimized adsorption energy between the metal sites with optimal bond length and oxygen-contained intermediates for enhancing the OER performance.

Electrochemical Measurements
All electrochemical measurements were carried out by an electrochemical analyzer (CHI 760E) in a typical threeelectrode cell.A saturated calomel electrode (SCE, CH Instruments) was used as the reference electrode, and a graphite rod was used as the counter electrode.The potential was converted to reversible hydrogen electrode (RHE) via a Nernst equation (E RHE = E SCE × 0.244 V + 0.0591 × p H).To evaluate the OER activities, the scan rate of linear sweep voltammetry (LSV) was set as 1.0 mV s −1 with the potentials from 1 to 1.8 V vs. RHE in 1.0 M KOH.Electrochemical impedance spectroscopy (EIS) was measured at 1.5 V vs. RHE with a frequency range from 10 5 to 0.01 Hz.All polarization curves were calibrated with iR correction unless noted.Cyclic voltammetry cycles (CVs) at 0.96-1.06V vs. RHE with the scan rates from 10 to 50 mV s −1 were applied.

PEC-OER Measurements
The PEC-OER measurements were conducted with a frontside illumination in all cases (light enters from the absorber side).A saturated calomel electrode (SCE, CH Instruments) and a Pt wire were used as reference electrode and counter electrode, respectively.The recorded potential vs. SCE (E SCE ) was converted against RHE using the Nernst equation (E RHE = E SCE × 0.244 V + 0.0591 × pH).A 1.0 M potassium borate buffer, prepared by adjusting the pH of 1.0 M H 3 BO 3 to 9.0 with KOH solution, was used as the electrolyte.The stability of photoanode was evaluated by measuring curves of the potential vs. time at 1.0, 2.0, and 2.5 mA cm −2 .

Computational Methods
All spin-polarized density functional theory (DFT) calculations were performed with Vienna Ab initio Simulation Package (VASP).In this work, the generalized gradient approximation (GGA) within Perdew-Burke-Ernzerhof (PBE) method was used to describe the exchange-correlation interaction.The core electrons were replaced by the projector augmented wave (PAW) pseudopotential, and 450-eV plane-wave expansion was setup for energy cut-offs.The k-points sampling was set to 1 × 1 × 1 Monkhorst-Pack k-points grid for geometrical optimization and that of electronic property calculation was 1 × 1 × 2. The convergence threshold was set to 10 -5 eV and 0.05 eV Å −1 for energy and force, respectively.Weak interaction was described by DFT-D3 method using empirical correction in Grimme's scheme.
The (1 × 2) surface of bulk CoNDA (100) was chosen, which contains 16 Co atoms, 83 O atoms, 96 C atoms, and 64 H atoms.A vacuum slab of about 15 Å was maintained in the super-cell configuration that was large enough for the calculations.To simulate the tensile state of structure, tensile CoNDA structure was constructed by expanding the lattice parameters of the pristine CoNDA in three directions by 5%.-S5) exhibited that the AE-CoNDA and control samples uniformly coated on the nanoworm-like BiVO 4 surface.Especially, the HR-TEM image of AE-CoNDA@BiVO 4 (Fig. S2) exhibits a lattice fringe of 1.18 nm for the interlayer spacing distance of naphthalene-based MOF with transition metal nodes along the [100] direction, which assigned to AE-CoNDA.In addition, in XRD pattern (Fig. S6), the diffraction peak below 10° stands for crystalline MOF structure, indicating that the AE-CoNDA is highly crystallized.Raman spectra (Fig. S7) also show obvious characteristic peaks which assigned to naphthalene-based MOF.Above results confirmed that AE-CoNDA was highly crystallized and successfully deposited on BiVO 4 photoelectrode.

Vital Role of AE-CoNDA as Cocatalyst
To probe the crucial role of AE-CoNDA as cocatalyst, the OER performance of AE-CoNDA was further evaluated in a three-electrode cell in 1.0 M KOH solution.The control samples with different reaction conditions were also synthesized to investigate the contribution of acetic acid etching.The optimal etching time was found to be 1.0 h, and optimal concentration of acetic acid was 0.2 mM (Figs. S16-S18).As shown in Fig. 2a, the as-prepared AE-CoNDA delivered only a low overpotential of 260 mV to reach a current density of 10 mA cm −2 .Compared to the CoNDA without acid etching (360 mV), this result highlights the significant role of acid etching in enhancing the OER performance.Notably, the overpotential for the AE-CoNDA was even smaller than that of the benchmark Ir/C (330 mV) at the same current density, demonstrating the excellent OER activity for the former.The fast reaction kinetics of AE-CoNDA was further underscored by its smaller Tafel slope (62 mV dec −1 ) than that of CoNDA with 82 mV dec −1 , and even commercial Ir/C with 97 mV dec −1 , respectively (Fig. 2b).The AE-CoNDA further exhibited a much smaller electrochemical resistance (R ct ) (Fig. S19 and Table S2) than that of the CoNDA, implying a fast charge transfer for AE-CoNDA.The ratio of inner charge to outer charge density for AE-CoNDA was calculated to be 94.5 (Figs.2c and S20), which was much higher than that of CoNDA (52.7), as a result of the high electrochemical porosity.These results suggested that the acid etching not only increased the electrode roughness [37], but also produced abundant exposed active sites.The larger electrochemically active surface area (ECSA) of AE-CoNDA than that of CoNDA indicated the more electroactive surfaces for AE-CoNDA (Figs.S21 and S22).Moreover, the turnover frequency (TOF) of AE-CoNDA was calculated to be 18.66 h −1 at an overpotential of 0.3 V, which was approximately three times higher than the corresponding value for CoNDA (5.86 h −1 ) (Fig. 2d).Above results claimed that the increase in active surface area is not the main reason for the increase in the current density of the catalyst, but the intrinsic characteristics of the catalyst promote the catalytic activity of the catalyst, resulting in a higher current density.
To further validate the rapid charge transfer for AE-CoNDA, in situ EIS measurements were conducted (Table S2).The results showed that the R ct value of AE-CoNDA was much lower than that of CoNDA over a wide range of potential (Fig. 2e), indicating the fast charge transfer efficiency for AE-CoNDA.Besides, the R ct value of the AE-CoNDA became stable at low potential (1.4 V), demonstrated that the OER process was triggered at low potentials on the AE-CoNDA [38,39].Notably, the excellent alkaline OER performance in terms of the overpotential (260 mV at 10 mA cm −2 ) and Tafel slope (62 mV dec −1 ) for the AE-CoNDA was superior to those of almost all non-carbonized Co MOF-based OER electrocatalysts previously reported (Table S3).Moreover, the OER process on the AE-CoNDA was stable, no apparent change in the overpotential was observed over 100 h of continuous operation (Fig. 2f), which is much stable than CoNDA under same potential (Fig. S23).
Especially, the XRD pattern (Fig. S24a) and Raman spectra (Fig. S24b) of AE-CoNDA after OER process remained similar curves with pristine AE-CoNDA, indicating structural stability of catalyst.

Analysis of Intrinsic Structure on AE-CoNDA
To gain the insights of superior OER activity on AE-CoNDA, we first performed XPS measurements.In the highresolution Co 2p, O 1s, and C 1s XPS spectra (Figs.3a, b and  S25), the Co 2p XPS peaks in the AE-CoNDA displayed a higher binding energy, shifted approxiately 0.62 and 0.40 eV, while O 1s XPS peaks positively shifted 0.06 eV to higher binding energy, as the peaks of O-C = O and C-C/C = C negatively shifted to lower binding energies (Fig. S25).These results demonstrated the electrons from Co atoms within AE-CoNDA transferred to the O atoms adjacent to C atoms and ultimately located on the C atoms.To deeply investigate the fine structure of AE-CoNDA, we further conducted X-ray absorption spectroscopy (XAS).The obtained X-ray absorption near-edge structure (XANES) spectra (Fig. 3c) showed the absorption edge with an increased photon energy of Co species within the AE-CoNDA in comparison with CoNDA and Co foil, indicating a higher valence state for Co species in AE-CoNDA.Moreover, the Co L-edge XAS spectra of the AE-CoNDA (Fig. 3d) exhibited a Co absorption edge shifted to a higher photon energy relative to that of the Co foil and CoNDA, but close to that of reference CoO, suggesting the average Co valence state of + 2 for AE-CoNDA.
Figure 4a shows the corresponding Fourier transforms of extended X-ray absorption fine structure (EXAFS) of AE-CoNDA, which reveals the major peak located at 1.6 Å attributable to the nearest shell coordination of the Co-O bond.It is noteworthy that the Co-O peak of AE-CoNDA shifted toward a higher R direction by about 0.05 Å relative to that of CoNDA, indicating that the bond length of Co-O in the AE-CoNDA was stretched [35].The stretched Co-O bond is ascribed to partial destruction of CoNDA structure and weakened interaction between Co cations and NDA ligand after acid etching.The EXAFS fitting was performed for the coordination shells in the R range of 1.0-3.0Å. Amplitude reduction factor S 0 2 as determined from fitting was 0.85.The EXAFS fitting results (Table S4) validated that the coordination number of Co-O bonds was determined to be smaller than 6.Additionally, 1 3 as revealed by the EPR spectra (Fig. S26), an apparent signal at g-factor value of 1.86 was observed for AE-CoNDA, whose intensity was stronger than which of CoNDA, indicating more uncoupled Co centers as defects in the framework in AE-CoNDA [40].EXAFS fitting result and EPR results proved distorted octahedral configurations and unsaturated coordinative Co species of CoO 6 centers in AE-CoNDA.
To gain more insights into the electron configuration and spin state of AE-CoNDA with stretched Co-O bond length, the O K-edge XAS spectra were further analyzed.As shown in Fig. 4b, the O K-edge XAS spectra consisted of two characteristic peaks between 532.0 and 534.0 eV, arising from the hybridization between unoccupied O 2p and Co 3d orbitals.Clearly, the intensity of O K-edge from AE-CoNDA was enhanced after acid etching, indicating an increased unoccupied density of states and a strengthening orbital hybridization of O 2p and Co 3d orbitals.Such enhanced intensity in O K-edge spectra, accompanying with the Co L-edge peak for AE-CoNDA (Fig. 3d), suggests an electron reconfiguration occurred in both the O 2p and Co 3d orbitals.
It is important to notice that the electronic configuration of Co species primarily dominated the spin state of Co species.To deeply explore the spin state of Co species within AE-CoNDA, we further carried out the zero-field cooling (ZFC) temperature-dependent magnetic susceptibility tests with a magnetic property measurement system (MPMS-VSM) [41].The total effective magnetic moment (μ eff ) of AE-CoNDA was obtained through χ −1 -T liner fitting to calculate the e g occupancy.In Fig. 4c, the calculated μ eff for AE-CoNDA and CoNDA was 3.667 and 1.935, respectively, and the number of d-orbital unpaired electrons can be further calculated by formula [42].Thus, the e g occupancy of Co species raised from approximate 0.98 to 1.84 after Co-O bond length stretched.The calculated e g occupancy indicated more unpaired d electrons existed in the AE-CoNDA (Fig. 4d), which induced a spin state transition from an intermediate spin (e g 1 t 2g 6 ) to a high spin (e g 2 t 2g 5 ) of AE-CoNDA [43].

Theoretical Evaluation of the Enhancement Effect on OER Activity for AE-CoNDA
To give an in-depth understanding of the relationship between the spin state of Co active sites and adsorption ability of oxygen-contained intermediates, we performed DFT calculations.Firstly, a CoNDA model with short bond length and AE-CoNDA model with long bond length were established (Fig. S27).As shown in Fig. 5a, the Co active sites adsorbed oxygen-contained intermediates of OH*, O*, and OOH* through hybridization between the O 2p and Co 3d orbitals throughout the OER process.Considering that the high spin state of Co active sites was beneficial for Co 3d and O 2p orbital hybridization to facilitate the adsorption of oxygen-contained intermediates [44], we also calculated the spin-resolved density of spin (DOS).The 3d-orbital of Co sites was splitted as dx 2 -y 2 , dz 2 , dxy, dxz, and dyz orbitals in Fig. 5b and c.As for AE-CoNDA (Fig. 5b), the state density of dz 2 (close to the Fermi level) was much stronger than that of CoNDA (Fig. 5c), demonstrating that more electrons transferred between different orbitals in AE-CoNDA.The overlapped area between the Co dz 2 orbital and O 2p orbital for AE-CoNDA was larger than that for CoNDA, indicating an enhanced interaction between the Co active sites and oxygen-contained intermediates after the Co-O bond length stretching.These results demonstrated that the bond length stretching facilitated the electron transfer from the t 2g orbitals to the e g orbitals, thus leading to a high spin state [40].
As displayed in Fig. 5d

Conclusion
We have developed a highly active AE-CoNDA catalyst with a stretched Co-O bond length and a high spin state.

Fig. 2
Fig. 2 Electrocatalytic OER performances of AE-CoNDA.a Polarization curves and b Tafel slopes of AE-CoNDA, CoNDA, and Ir/C.c Internal and external voltammetric charge density and electron porosity of CoNDA and AE-CoNDA.d TOF values and e response of R ct at different potentials for CoNDA and AE-CoNDA.f Chronopotentiometric durability of AE-CoNDA at 1.5 V

Fig. 3
Fig. 3 Electronic structure analysis of AE-CoNDA.High-resolution a Co 2p and b O 1s spectra of CoNDA and AE-CoNDA.c Co K-edge XANES spectra of AE-CoNDA, CoNDA, and Co foil.d Co L-edge XANES spectra of AE-CoNDA, CoNDA, CoO, and Co foil , the AE-CoNDA model showed an accumulated charge of O center in NDA linker from Co active sites with the stretched Co-O bond length compared to control CoNDA, indicating a rapid electron transfer for AE-CoNDA [45].The specific free energy (ΔG) for each elementary step was calculated to estimate their OER activity at the center Co sites.In Fig. 5e, it can be clearly seen that the elementary steps for the formation of OH* and transition of OOH* to O 2 desorption were down-hilled, while other reaction steps involved in the OER process for AE-CoNDA and CoNDA were uphilled.Based on its ΔG diagram, the ΔG value for O* formation at the center Co sites decreased from 1.41 eV for CoNDA to 1.26 eV for AE-CoNDA, revealing that the spin state transition from intermediate spin to high spin led to a decrease in

Fig. 4
Fig. 4 Fine structure analysis and spin state transition of AE-CoNDA.a Fourier transformed EXAFS spectra of AE-CoNDA, CoNDA, and Co foil, inset: EXAFS fitting curves in R space of AE-CoNDA and CoNDA.b O K-edge XANES spectra of AE-CoNDA, CoNDA, and CoO.c Temperature-dependent inverse susceptibilities of AE-CoNDA and CoNDA by the susceptibilities derived from the magnetizations (χ = M/H) follows by Curie-Weiss law.d An illustration of 3d-orbitals of AE-CoNDA and CoNDA The AE-CoNDA required only a low OER overpotential of 260 mV to reach 10 mA cm −2 with a long-term stability.The AE-CoNDA and BiVO 4 integrated photoanode achieved a photocurrent density of 4.30 mA cm −2 at 1.23 V under AM 1.5G irradiation, superior IPCE of 54.0%, ABPE of 1.61%, and prolonged durability in base.Extensive structural characterization verified that the spin state of the center Co sites within the AE-CoNDA was transferred from an intermediate spin state (e bond length stretching.The DFT calculations disclosed that the high spin state of AE-CoNDA enhanced the Co 3d and O 2p orbital hybridization, promoting the reaction kinetics for the RDS (OH* → O*).The in situ electrochemical spectroscopic results demonstrated the bond stretching in AE-CoNDA accelerated the OH*

Fig. 5
Fig. 5 Investigation on correlation between spin states of AE-CoNDA with OER activity.a An illustration of AE-CoNDA during OER process including the Co active sites and oxygen-contained intermediates adsorption.Calculated spin-resolved DOS of b AE-CoNDA and c CoNDA.d Calculated charge density difference of AE-CoNDA and CoNDA.The yellow and cyan regions represent electron accumulation and depletion.e The free energy diagrams of CoNDA and AE-CoNDA of OER